What is the Ambient Color in the Blender Internal Renderer exactly?
Ambient Color: The Ambient color is a constant color added to the reflection model. Light sources are summed together and used in the calculation of the diffuse and specular part (See Phong reflection model for details)

phong reflection = diffuse + specular + ambient


Horizon Color: The color to use when the raytracer does not find any intersection for the primary ray (direct ray from the camera). It does not influence the shading at all. You can see this as a sort of background underneath your render.

Zenith: When using sky_blend the background will blend from the Horizon Color to the Zenith. This is done by the output pixel coordinate so the result will be the same for all camera angles and positions.

So the Horizon Color and Zenith, that generates the background, can be seen as an underlay.